Such amplifier circuits are widely used particularly in digital data processing systems and, more specifically, they are employed in memory systems where, during read-out, the bit storage cells deliver a signal representative of the stored binary value, "1" or "0", which signal needs to be properly recognized and amplified to obtain a digital signal of a distinct level for further processing.
Numerous sense amplifiers, particularly amplifiers for use in semiconductor memory systems have been proposed, all designs aiming at satisfactorily meeting the essential requirements for an amplifier that can be used in today's high density, high speed memories. These amplifiers need to be reliable, fast, sensitive to small signals and they should be of simple structure requiring only a minimum of space on a semiconductor chip. In addition, the circuit must meet these requirements without demanding extremely tight device margins since the unavoidable fabrication process tolerances, which vary widely depending on the technology used, may otherwise not permit the use of high-volume manufacturing processes. Particularly critical in this regard are current gallium arsenide (GaAs) technologies.
A sense amplifier arrangement for use in a semiconductor memory that is typical for today's state of the art has been disclosed at the IEEE GaAs IC Symposium held at Boston, Mass., Oct. 23-25, 1984. Article "A GaAs 4K BIt static RAM with Normally-on and -off Combination Circuit", appearing on pages 117-120 of the Technical Digest of that conference describes a memory system including a storage cell matrix and a sense amplifier which is of the differential amplifier type. Its inputs are connected to the pair of bit lines associated with the storage cells that are represented by the well-known and widely used "6 device cells". So-called `pull-up` field effect transistors (FET's) serve to bias the lines and to remove any charges on the line before starting the next read operation.
Currently, differential sense amplifiers are frequently employed but this type of amplifier generally needs a second power supply voltage, the circuitry is more complex than that of regular amplifier circuits and, furthermore, it is a true analog circuit which is more difficult to realize than digital circuits, particularly in GaAs technology.
In the article "Estimation of GaAs Static RAM Performance", published in IEEE Transactions on Electron Devices, Vol. ED-29, No. 7, July 1982, pp. 1130-1135, another random access memory (RAM) system is described in which a regular amplifier is provided for each bit line. In contrast to the operation of the differential amplifiers, the regular amplifiers do not compare the two signals appearing on a bit line pair but compare the received signals with a built-in reference. This becomes particularly critical when the signal to be sensed is small compared to the bias voltage that is applied to the bit line. When the reference and the bias voltage do not track, i.e., when deviations from the optimum nominal value are not self-compensated, the realization of amplifiers providing the required reliability becomes critical with today's manufacturing tolerances when several amplifiers are required on the same chip.